Decay
Decay, the breaking down, or decomposing, of dead tissue or organic matter into simpler chemical compounds. Many kinds of bacteria and such fungi as molds and yeasts bring about decay. These organisms, called saprophytes, grow and multiply on nonliving organic matter that they use as food. They release enzymes that chemically break down the organic matter into tiny nutrient particles, which they absorb. Usually, decay occurs most rapidly in warm, moist air, and away from direct sunlight. Any change in these conditions either slows down the action of saprophytes or kills them. Methods for protecting food against decay are discussed in the article food preservation.
Some types of decay are beneficial. Bacteria in the soil change certain nitrogen compounds into ammonium compounds that are necessary for plant growth. Other saprophytes break down dead plants and animals into simple chemicals. For example, coal and petroleum are formed from plants that became buried in swamps and decayed. Decay-producing bacteria are used in cheese making, hide tanning, tobacco curing, and the processing of flax and hemp.